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ProcessSafetyCalculations RenatoBenintendi MScCEngFIChemE
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Preface Theaimofthisbookistoprovidethereaderwithsomeguidanceoncalculationsinprocess safety.Accordingly,theintentionoftheauthorwasnottoduplicateortoemulatethe manyexcellentliteratureworksproducedsincethemanyyearsofstudyonprocesssafety techniquesandmodels,butrathertobuild-upalogicalandfluidthreadtoovercomedoubts, uncertainties,anddifficultiesoftenmetincalculationexercises.Theavailableliterature sourcesoffereitherabroadrangeofdifferentmodelsandapproachesor,evenwhentheyare calculationsoriented,sometimesunavoidablyandfaultlesslyleavesomegapsinthecalculation criteria;thisisafeetobepaidtotherichnessandvarietyofdataandinformation.This bookhasadifferenttarget:toprovideaclearindicationonwheretogoinpracticalapplications whenacrossroadsismet,andwhenavailabledataaredifficulttobeconvertedintofiguresand findings.Nevertheless,thetheoreticalandconceptualbackgroundisdeemedtobeeffective inenablingtheusertoproperlyframethetopicsand,tosomeextent,someaspectsnotincluded intheexistingliteraturesourceshavealsobeendealtwith,fromprinciplestoapplications.
Thebookisthefinalstepofalongtriptheauthorstartedin1988,when,morethan10yearsafter theincidentsofFlixboroughandSeveso,andsomeyearsaftertheunresolvedtragedyof Bhopal,theSevesoIdirectivewasactuatedinItaly.ItisdoubtlessthatthisEuropeanlegislative acthasgivenatremendousimpulsetothedevelopmentofsystematicmethodsandtechniques inprocesssafetyengineering.Intheninetiestheauthorwasinvolvedasateacherinthefinal partoftheChemicalPlantscourseheldattheChemicalEngineeringfacultyattheUniversityof Salerno(Italy),providingsomeguidanceaboutprocessriskassessmentmethodologies.Inthe sameyears,alongexperienceacquiredasaninstructorwithinthecourseforRiskAnalysis, managedbytheItalianInspectoratesengagedintheSevesoDirectivesafetyreportsassessment, clearlyshowedhowdifficultandchallengingitwastorelatetheorytorealcases.Specifically, evenifchemicalengineers,andengineersingeneral,shouldhaveathoroughknowledgeof backgroundconceptsunderpinningprocesssafetystudies,theexperiencehasshownthat culturaltransitionfromprocesstoprocesssafetyengineeringisneitherautomaticnoreasy.The authorhasanalysedthisaspectinarecentarticle(Benintendi,2016),wherehehaspointedout thattheeffectivenessof adding-on basicprocesssafetyconceptstotheuniversitybackgroundis notalwayshigh.Thecombinationoftheexperienceacquiredfromprocesssafetyteaching, tutoring,andlecturingatseveraluniversitiesintheUK,Italy,Asia,andUSA,withthe
professionalexpertisedevelopedinalmost30years’work,hassuggestedthattheprovisionof basicconceptsalreadycalibratedonprocesssafetyismuchmoreeffective.
Inthisrespect,thisbookincludesafirstpartwherebasicconceptsofchemistry, thermodynamics,reactorengineering,hydraulics,andfluid-dynamicsarereviewedwitha specificfocusonprocesssafetyscenarios.Dozensoffullyresolvedexamplesfocusingon processsafetyapplicationshavebeenincluded.This Fundamentals sectionendswithone chapterdealingwithstructuralanalysisforprocesssafetyandanotheroneincludingastatistics andreliabilityoverview,aimingtoprovidethebasicconceptstoproperlymanagethe probabilisticaspectofriskassessmentstudies.Allthesefirstchaptersincludemanyliterature data,withtheintentiontoprovidetheuserswithacompletetoolfortheircalculations.
The ConsequenceAssessment sectionisorganisedaccordingtothetypicalsequentialoutcomes followingareleaseafterlossofcontainment.Someeffortshavebeenmadetoensurethatall potentialgapsanduncertaintiesinthecalculationswerecoveredandovercome,basedonthe professionalinvolvementoftheauthorinmanyprojectsdealingwithoilandgas, petrochemical,pharmaceutical,finechemistry,food,andenvironmentalsubjects.Inthis respect,theuserswillbedrivenacrossarelativelysimpleanddirectroute,unlikewhathappens whentheygototheliterature,whereobviouslyamuchwiderspreadofmethodsisprovided. Chapter7 focusesonreleasesfromcontainmentsandfrompools:onthebasisofthetheoretical backgroundprovidedinthe Fundamentals section,asystematicanalysisofpossiblescenarios hasbeencarriedout,withthesupportofmanyfullyresolvedexamples.Releaseofcarbon dioxidehasbeendealtwithindetail,duetotherelativelynewhazardousscenariospresented aftertheintroductionofCarbonCaptureandStorage(CCS)process,andtothespecificnature ofthissubstance,whichshowsasolid-liquidequilibriumbelowthetriplepointanddoesnot fullybehaveaccordingtoequilibriumthermodynamics. Chapter8 presentsdispersionmodels; intheauthor’sintention,theefforthasbeenmadetoresolvethevariousuncertaintiesmetby processsafetyengineersonwhichmodeltoadopt,whichregimetoselect,whichphaseofthe dispersionroutetoidentify,andwheretolocaliseit.Keyparametershavebeenidentifiedto drivethisapproachwiththesupportofmanyexamples.Eisenberg’smodelforflashfireand Kalghatgisolid’sflamemodelforjetfirehavebeenselectedfortheirsimplicity,completeness, androbustnessin Chapter9,whichcoversfire.Aspecificfocushasbeenmadeonignition sources,accordingtothesystematicBSEN1127-1standard,withtheaimtoreducethe incompletenessoftheapproachoftenfollowed. Chapter10 dealswithgasandvapour explosions,consistingofallofscenariospotentiallyresultinginsignificantoverpressures, includingBLEVE,RapidPhaseTransition,andthermalrunaway.TheMultiEnergyMethod (MEM)hasbeenfittedwiththefindingsoftheGAMEprojects,andthishasbeenveryeffective inremovingthetraditionallargelevelofsubjectivityanduncertaintyinblastcurveselection.A MEMdetailed,andfullyresolved,examplehasshownaverygoodconsistencywiththe findingsoftheBaker,Strehlow,andTang(BST)method. Chapter11 hasbeenincludedto coverdustexplosions.Inadditiontothemodelsdescribingtheprimaryandthesecondary
explosions,someHAZIDcasesrelatingtodustprocessingequipmenthavebeenincluded, accordingtothegreatemphasisthemachineryandtheATEXdirectiveshaveputonthis specificaspect.AcasestudydealingwiththeImperialSugarCompanyhasbeenanalysedand verifiedagainstsomecalculationfindings.
Chapter12 dealswithQRAtechniques,includingtheexceedancefrequencycurvebuild-up,the ALARPmodeldemonstration,theFNcurves,andthepartscount.Someapplicationshavealso beengiveninthischapter.
Inthisbook,unlessotherwisespecified,allunitsareexpressedaccordingtotheInternational System(SI)ormkssystem.Thisbookaimstosupportscientistsandengineersworkingin processsafetyengineering.Itisworthrepeatingthatitisabookofcalculationsofferingalarge numberofdatausefulforthispurpose.Theauthorguessesthatitisnotfreefrommistakesand defects,andtheauthorapologiesinadvanceforthat.Hewillbegratefulforanycontributions readerswillwishtogivehim,toensurethattheobjectivesthewriterhadinhismindcanbefully achieved.
Reading(Berkshire),30April2017
Reference
Benintendi,R.,April2016.Thebridgelinkbetweenuniversityandindustry:akeyfactorforachievinghigh performanceinprocesssafety.Educ.Chem.Eng.15,23–32. IChemE,Elsevier.
Acknowledgements Iwishtothankvariousprofessors,colleagues,andfriendsfortheircontributiontothisbook: ProfGianniAstaritaofUniversityofNaples(Italy)andDelaware,whoprovidedmewiththe mysteriouscodesofchemicalengineering.Thisbookisdedicatedtohismemoryasan appreciationfortheprestigiousChemicalEngineeringSchoolhecreatedinNaples,thatIhad thehonourandthepleasuretoattend.SimonaRega,forherprecioussupportandforthe contributiontothedevelopmentoftheRapidPhaseTransitionPhaseincludedinthisbook.My colleagues,FosterWheelerandAmecFosterWheeler,Readingoffice,whoinspiredthiswork throughtheirjointactivityandthecommitmentoftheProcessSafetyCalculationscourseheld inReadingin2014.Mystudentsofthemasterinprocesssafetyengineeringattendedat Sheffield,Leeds,andParis,whomItutored,givingmetheopportunitytomakeamuchbetter focusonthesubjectfromthisstandpoint.TheteamoftheProjectEvaluationLaboratoryofthe UniversityofSalerno(Italy),withwhomIamsharingandextendingtheriskassessment techniquesinamuchwiderperspective,whichhasresultedinasharperfocusonmethodsand philosophy.
Finally,Iwouldliketoexpressmythanks,gratitude,andappreciationtotheElsevierteamfor theirsupportandpatience:FionaGeraghty,AnitaKoch,RenataRodrigues,andMariaConvey, withoutwhoseadvicethesepageswouldhavebeenneitherwrittennorpublished.
Reading(Berkshire),30April2017
ChemistryofProcessSafety Nothingislost,nothingiscreated.Everythingistransformed. (A.Lavoisier)
1.1StoichiometryandMassBalances Stoichiometry(fromancientGreek στ οιχε ~ ιον element and μέτρον measure)isafundamentalpart ofbasicchemistrythatcanbedefinedas therelationshipbetweentherelativequantities ofsubstancestakingpartinareactionorformingacompound,typicallyaratioofwholeinteger. Thestoichiometryofachemicalreactioncanbeexpressedthroughtheexpression:
or
where a, b, c, d arethereactantsandproductsreactioncoefficients,and A, B, C, D arethe atomsorthemoleculesinvolvedinthereaction.Eq. (1.2) isageneralisedformofthesame formula.Itisworthnotingthat:
isthestoichiometricratioof Ri to Qj andindicatestheconstantrelationshipbetweenthe concentrationsofthesechemicalcompoundsthroughoutthereactionpath.
Puremixinganddispersionprocessesdon’tcauseanymodificationsofthechemicalidentityof thesubstancesbecauseofthephysicalnatureofthesephenomena.
1.1.1MassBalances Massbalancesarebroadlyusedinprocessengineeringwhereprocessstreamsandconfigurations arewelldefined.Instead,processsafetyscenariosarevariableandoftenverycomplex. Consideringaportionofspace(Fig.1.1),thefollowinggeneralequationofmassbalanceapplies:
¼ Win Wout + Wgen
ProcessSafetyCalculations. https://doi.org/10.1016/B978-0-08-101228-4.00001-0 # 2018ElsevierLtd.Allrightsreserved.
Massbalanceonagenericspacedomain.
where:
- Win isthemassenteringthespacedomain.
- Wout isthemassleavingthespacedomain.
- Wgen isthemassgeneratedorconverted.
- Wacc isthemassinventoryvariation.
AccordingtoLavoiser’sprinciple, Wgen existsonlyforcomponentswhicharetransformedinto others.
Processsafetyengineeringentailsabroadrangeofcomplexandvariablescenarioswherefull understandingofstoichiometryandmassbalancesisnecessarytoproperlyanalyseandassess therelatedprocessandplantconfigurations.Somecasesarediscussedhere,andspecific scenarioshavebeenanalysedinthenextparagraphs.
1.1.2ChemicalReactions
Alowpressurevesselcontainsastoichiometricmixtureofcarbonmonoxideandpureoxygenat ambienttemperature To (Fig.1.2).
Thesystemundergoesachemicalreactionthatconvertsallcarbonmonoxideintocarbon dioxideandisassumedtobeatthermalequilibriumsothatinitialambienttemperatureis attained.Applicationofidealgaseslaw,with V and To constant,gives:
where N1 and No arethefinalandinitialnumberofkmolesofproductandreactantsrespectively, whichinthisspecificcasecoincideswiththereactionstoichiometriccoefficient srp:
Fig.1.1
Itcanbeconcludedthereaction,assuminganoverallisothermalandisochoriccondition, causesa33%pressuredrop,whichcouldresultinacatastrophicoutcomeforthevessel.
1.1.3JetFlowsFromPressurisedSystems Jetflowsfrompressurisedcontainmentsarefrequentinprocesssafety.Theconsequencesof toxicorflammablecompounddispersionstrictlydependonthejetdynamics.Thescenario shownin Fig.1.3 illustratesthereleaseofhydrogensulphidefromapipeline.
Thetoxicgasisreleasedwithamassflowrateof WH2 S .Airisentrainedintothejetaslongas thisisdeveloped,resultinginaprogressivedilutionofH2S.Dependingontheeffectofthe entrainment,toxicconcentrationsareproportionallyreduced,whileflammabilitywillbe promotedbyairmixingwithinaspecificregionofthejet.Assumingasteadystatevalueof
Fig.1.3
Fig.1.2
Oxidationofcarbonmonoxideleadingtovesseldepressurisation.
WH2 S ,andindicatingwith WAIR(z)theairentrainmentmassflowrateperlengthunitalong z,the massbalanceat z ¼ h canbewrittenas:
Ithasbeenshownhowimportantthecorrectmanagementofthisbalanceisinjetflow consequenceassessmentstudies.
1.1.4FlashFlow Aflashflowisthereleaseofaliquidfromacontainmentwheretheoperatingtemperatureis significantlygreaterthanitsdownstreamboilingtemperature,typicallythenormalboiling temperature.Theliquidisforcedtovaporiseafractionofittoreachthedownstream equilibriumcondition.ThisisthecasewithLPGstoredatambienttemperature(Fig.1.4).
Theliquidmass W splitsintotheflashedvapourfraction XV andtheliquidfraction XL.Itis:
1.1.5AbsorptionandAdsorption Removaloftoxicordangerouscompoundscanbeaccomplishedviamasstransferunits,suchas absorberoradsorptiontowers.Atypicalexampleistheaminetreatmentofsourgas(Fig.1.5), orabsorptionofcarbondioxidewithsodiumhydroxide.Forsourgastreatment,neglecting changesofflowrates Q and q,themassbalanceofH2Scanbesimplifiedconsideringthe concentrationofsulphur S
gas Q, Sin
gas Q, Sout
Example1.1 Howmanykilogramsofoxygenarerequiredtoenrich500kgofairto50%ofO2 (molarbasis)? (Airmolecularweight ¼ 29,oxygenmolecularweight ¼ 32)
Solution 500kgofaircorrespondtothefollowingkmoles:
Thefinalamountofoxygenwillbe,asrequested,thesameasthatofnitrogen,i.e.13.62kmol. Therefore,theoxygentobeaddedcanbecalculatedbydifference:
Example1.2 Agasstream,otherthanair,containingacertainamountofhydrogensulphideissenttoaburner thatisfedwiththenecessaryairtooxidisethesulphidetosulphurdioxide.Knowingthattheoff gascontains464.8kg/hofmolecularnitrogenand20.8kg/hofmolecularoxygenandthatthe sulphideistheonlyoneoxidisablegas,determine:
(a)Themassflowrateofhydrogensulphidesenttotheburner
(b)Thesulphideoxidationefficiency
Fig.1.5 Aminetreatmentofsourgas.
(Hydrogensulphidemolecularweight ¼ 34,nitrogenmolecularweight ¼ 28)
Solution
Withreferenceto Fig.1.6,thecombustionprocessisrepresentedbythefollowingreaction:
Nitrogendoesnotparticipateinthereaction,sobeingthestoichiometriccoefficient:
andthenitrogenmolarflowrate 468 4 28 ¼ 16.6kmol/h,thehydrogensulphidemassflowrateis:
Unreactedoxygenis0.65kmol/h.Oxygenfedtotheburnerisequalto 16:6 0 21 0:79 ¼ 4:4kmoles=h.Theconversionefficiencyiscalculatedconsideringtheratioof unreactedtosuppliedoxygen:
Example1.3
Astoragetankcontainsaheavyhydrocarbonwithanegligiblevapourpressureattheoperating temperature.Thetankisnitrogenblanketedsothatapositiveoverpressureof Po ismaintained. Thetankemptyingisstartedwithaheadfreevolumeof Vo andaliquidflowrateequalto Q.Find thenitrogenmasstobeprovidedwithtimebythecontrollerPICinordertoensurethatpositive overpressure Po ismaintainedduringallemptyingphases(Fig.1.7).
Fig.1.6
Emptyingofnitrogenblanketedtank.
Solution
Thetankheadspaceisassumedtobeoccupiedbynitrogenonly.Applicationofidealgaslaws:
ImposingthatpressureismaintainedconstantbythePCV.Itis:
Simplifyingandrearranging:
Consideringthat dV dt ¼ Q andthat V ¼ Vo + Q t ,Eq. (1.20) maybewrittenas:
Separating:
Solving:
where nN2o isthemolarnitrogenamountof Vo at t ¼ 0.Thisresultisintuitivebuthasbeen rigorouslyobtainedhereviatheapplicationofmassbalances.
1.2StatesofSubstancesinProcessSafety Substancesinprocesssafetycanbepresentinthefollowingforms:
1.2.1GasesandVapours Gasisafundamentalstateofsubstancesatatemperaturehigherthantheircritical temperature.Hydrogenandmethanehavetoberegardedasgasesatambient temperature,whereaspropaneandsulphurdioxidearevapoursandcanbecondensed bycompression.
1.2.2Liquid Liquidsarethecondensedphaseofvapours.Theycanbeinequilibriumwiththeirvapoursat anytemperature,andvapourpressureistheequilibriumpressureexertedbyvapourabove theliquids.Liquidscanbemiscibleorimmiscible,polarornon-polar,andthisbehaviour stronglyaffectsthereleaseanddispersionscenarios.
1.2.3Dusts Inadditiontobeingcombustible,dustswhicharefinelydividedsolidparticlescanbe explosive.Accordingto BS-EN60079-10-2:2015,combustibledusts,500 μmorlessin nominalsize,mayformanexplosivemixturewithairatatmosphericpressureand normaltemperatures.Particulartypesofsolidparticles,includingfibres,arecombustible flyings,greaterthan500 μminnominalsize,whichmayformanexplosivemixturewith airatatmosphericpressureandnormaltemperatures(BS-EN60079-10-2:2015).
Asforgasesandvapour,themechanismofdustexplosionconsistsoftherapidreleaseofheat duetothechemicalreaction:
Fuel+oxygen ! oxides+heat
Metaldustscanalsoexothermicallyreactwithnitrogenandcarbondioxideaccordingto Eckhoff(2003),whichclassifiesexplosivedustsasfollows:
-Naturalorganicmatters
-Syntheticorganicmaterials
-Coalandpeat -Metalssuchasaluminium,magnesium,zinc,andiron.
1.2.4HybridMixtures Ahybridmixtureisacombinedmixtureofaflammablegasorvapourwithacombustibledust orcombustibleflying,whichcanbehavedifferentlyfromthegas/vapourordustindividually (BS-EN60079-10-2:2015).
1.2.5ExplosiveMists
Disperseddropletsofliquidswhich,insomesituations,mayformaflammablemistwhich maythengiverisetoanexplosionhazard.Ithasbeenprovedthataerosolsizeddroplets (sub-micronto50microns)willlikelybethemosteasilyignitableportionofthemistcloud (BS-EN60079-10-2:2015).
1.2.6SupercriticalFluids
Thedefinedstateofacompound,mixture,orelementaboveitscriticalpressureand criticaltemperature(IUPAC,2014).CarbondioxideinCarbonCaptureandStorage(CCS) processingisfrequentlyhandledinitssupercriticalstate.
1.3MassandConcentrationUnitsinProcessSafety Mole
Amountofgramscorrespondingtotheatomicormolecularweightofasubstance
Kmole
Amountofkilogramscorrespondingtotheatomicormolecularweightofasubstance
Molarfraction(Gasandliquidphase)
xi ¼ molarfractionofcomponent i
Ni ¼ numberofmolesorkmolesofcomponent i
Partialpressures(Gasphase)
AccordingtoDalton’slaw,withinthelimitofvalidityofidealgaslaw:
yi ¼ molarfractionofcomponent i inthegasphase
pi ¼ pressureofcomponent i
P ¼ totalpressure
1.3.1PartialVolumes(GasPhase) AccordingtoAmagat’slaw,withinthelimitofvalidityofidealgaslaw:
yi ¼ molarfractionofcomponent i inthegasphase
Vi ¼ volumeofcomponent i
V ¼ totalvolume
1.3.2MassFraction(GasandLiquidPhase)
and:
xmi ¼ massfractionofcomponent i
mi ¼ partialpressureofcomponent i
wi ¼ massflowrateofcomponent i
1.3.3MasstoVolumeConcentration(GasandLiquidPhase)
where:
ci isthemasstovolumeconcentrationofcomponent i. V(T )isthesystemvolumeattemperature T.
Particularmasstovolumeconcentrationsare(gasphase):
where:
cNi isthemassconcentrationatnormalconditions.
cSi isthemassconcentrationatstandardconditions.
Bothstates,normalandstandard,areassumedinthisbooktobeat273.15Kand1atm,according to Hougenetal.(1954).Undertheseconditionsthenormalmolarvolumesareasfollows:
Volumeof1mol ¼ 22:414L
Volumeof1kmol ¼ 22:414m3
1.3.4PartsperMillion(GasandLiquidPhase)
ppmw(weight)—typicalinliquids
Ifliquidiswater,assumingwaterdensityas1000kg/m3 or1000g/L:
1.3.5PartsperMillion(GasPhase)
whereallsymbolsareknown.
1.3.6MolarConcentration(AqueousSolutions)
Itisdenotedas[X]andindicatesmoles/litre.
1.3.7ConcentrationUnitsConversionSummary
See Table1.1.
Example1.4
TheIDLH(immediatelydangeroustolifeandhealth)ofsulphursulphideis100ppmv.Calculate itasmg/Nm3 andasmolarfraction.(MW 34)
Solution
Table1.1Concentrationunitsconversionsummary
1,000,000 22 414 mg/Nm3 Molarfractiongas
:414 MW 1,000,000
mg/m3 Molarfractiongas 22:414 T 273 15 1,000,000 MW Molarfractiongas
414 T 1,000,000 MW
:414
273 15 MW 22:414 T
22:414 T
MW 273 15
MW,molecularweight; T,temperatureinK.
Example1.5
Apressurevesselcontains:
10kgofmolecularnitrogen (MW 28)
20kgofmolecularoxygen (MW 32)
50kgofcarbondioxide (MW 44)
Calculatemolarandmassfractionsat: T ¼ 20°Cand1atm T ¼ 60°Cand3atm
Solution
Molarandmassfractionsdon’tdependonpressureandtemperature.
1.4SolutionsandChemicalEquilibrium Asolutionisdefinedasahomogeneousone-phasemixtureoftwoormoresubstances.In processsafetyitisveryfrequenttodealwithmixturesandsolutionswhichmayexistsinanyof thethreestatesofmatter,gaseous,liquid,andsolid.Knowledgeofsolutionsandmixtures chemistryisimportanttoidentifyandcalculatehazardouspropertiesoftheinvolved substances.
1.4.1GaseousSolutions
Examplesofgaseoussolutionsaremixturesofgaseoushydrocarbons,airthatismainlya mixtureofoxygenandnitrogen,off-gasesfromflaring,whichtypicallycontainscarbon dioxide,sulphurdioxide,water,andnitrogen.Nonreactivegasmixturespresentahighdegree ofhomogeneity,sotheycanalwaysbeconsideredsolutions.Thisisnotalwaystrueforliquid andsolidmixtures.GaseousmixturesaregovernedbyDalton’slawofpartialpressures,that statesthat thetotalpressurePofamixtureofncomponentsiisequaltothesumofthepartial pressuresPi ofallthedifferentgases:
Example1.6 100kgofsolidsulphurareburntinacombustoratatmosphericpressure.Knowingthat10% ofairexcessisused,findthepartialpressureofnitrogenintheoffgas.(Sulphurmolecular weight:32,nitrogen:28,oxygen:32).
Solution
Thecombustionreactionis:
100kgofsulphurareequivalentto3.125kmol.Fromthereactionstoichiometryandconsidering 10%ofairexcess:
Thisresultisintuitive,duetheequimolarS/O2 ratio.
1.4.2KineticsandEquilibriuminGasReactiveMixtures ThegasphasereactionrateofthegeneralchemicalreactionpresentedinEq. (1.2) maybe writtenas:
where rf istheforwardreactionrate, kf isthekineticconstant,and pRi arethereactant’s partialpressures.Somereactionsmaybereversible,thereforeasimilarequationmaybewritten forthebackwardreaction:
Atequilibriumthetworeactionratesarethesame:
where KP istheequilibriumconstant.
Manyimportantreactivemixturesinprocesssafetyreachtheequilibrium.
1.4.3LiquidSolutions Liquidsolutionsareobtainedbydissolvinggaseous,liquid,orsolidsubstancesinliquids. Dependingonthenatureandthebehaviourofthedissolvedsubstances(solute),andofthe liquid(solvent),awiderangeofphysical–chemicalscenariosmaybeobtained,whichhaveto bewellunderstoodinorderforthemtobeproperlyanalysedprocesssafetywise.
Liquid–liquidsolutions
Miscibleliquidsformhomogeneoussolutions,whereasimmiscibleliquidsformtwophase dispersedemulsions.Ageneralcriterionusedtoestablishwhetherornottwoormoreliquids aremiscibleiscomparingtheirpolarfeatures.Theoldsaying likedissolveslike isaveryuseful ruleofthumb.Therefore,polarspecies,suchaswater,havetheabilitytoengageinhydrogen bonding.Alcoholsarelesspolar,butcanformhydrogenbondingaswell.Duetoitsstrong polarity,waterisanexcellentsolventformanyionicspecies.Non-polarspeciesdonothavea permanentdipole,andthereforecannotformhydrogenbonding.Organiccovalentliquids,such asmanyhydrocarbons,fallwithinthiscategory.
Thefollowinggeneralcriteriacanbeadoptedtopredictsolubilityofchemicals:
-Symmetricstructuremoleculeshaveaverylowdipolemomentandarenotdissolved bywater
-MoleculescontainingO HandN Hcanformhydrogenbonds
-Moleculescontainingfluorineandoxygenareexpectedtohaveahighdipolemoment -Purehydrocarbons,oilandgasoline,arenon-polarorweakmolecules
Dipolemomentgivesjustaverygeneralindicationofsolubilityofmolecules. Table1.2 includesthedipolemomentforsomeorganicandinorganicsubstances.
Acommonpracticeistoassumethefollowingscaleofpolaritywithrespecttothedipole moment:
-Dipolemoment < 0.4:Nonpolarmolecule.Behaviourequivalenttohomopolar covalentbond.
-0.4 < Dipolemoment < 1.7:Polarmolecule.Behaviourequivalenttoheteropolar covalentbond.
-Dipolemoment > 1.7.Verypolar(ionic)molecule.
Table1.2Dipolemomentforsomeorganicandinorganicsubstances Substance
Resins2–3c Ionic
Crudeoils <0.7c Non-polar/polar
Asphaltenes 4–8c Ionic
aDean(1999)
bPolingetal.(2001)
cRiazi(2005)
Vapour–liquidequilibriuminliquidsolutions
Aliquidmixturemaypresenttoxicorflammablecharacteristicsdependingonconcentrations ofitscomponents,bothintheliquidandinthevapourphase.Therefore,itisimportantto understandthebehaviourofsolutionsinbothphases.Themostgeneralequilibriumrelationship betweenvapourconcentrationandliquidconcentrationofasubstanceinamixtureisgivenby theequation:
where yA and xA arethemolarfractioninthevapour/gasphaseandintheliquidphase respectively,and KA isthedistributioncoefficient. KA isanexperimentaldatum,whichdepends onsystemtemperatureandpressure.Asimplebutapproximaterelationtodescribetheliquid–vapourequilibriumofamixtureisRaoult’s,whichstates:
Thepartialpressureofsolvent pA overasolutionequalstheproductofthevapourpressure ofthepuresolvent, PA o bythemolefractionofsolvent, xA,inthesolution.
and,accordingtoEq. (1.47),if yA isthevapourmolarfractionand P thetotalpressure:
Themoredilutethesolution(ahighfractionofsolventtypicalofsocalledidealsolutions)the moreaccurateRaoult’sassumption.
Example1.7 Amixtureof n-butaneand n-pentaneisinequilibriumat2atme30°C.Determinetheliquidand vapourcomposition(Vapourpressures: n-butane ¼ 3.2atm, n-pentane ¼ 0.78atm)
Solution
Fourequationsareavailablewithfourunknownvariables.
Solving:
Anempiricalcorrelationtocalculate KA forhydrocarbonsandsomeinorganicgasesis Hoffman’sequation(Hoffmanetal.,1953):
Table1.3ParametersofHoffman’sequation
and b and TB (normalboilingtemperature)areincludedin Table1.3.
ForC7+ fractionsthefollowingequationscanbeused:
Pressureisgiveninbar.
1.4.4AzeotropicMixtures ApplicationofRaoult’lawshowsthatmixturesofvapourcompositionaregenerallydifferent fromliquidcomposition,duetothedifferentvolatility(vapourpressures)ofthecomponents. Thisisnotalwaystrue,becausesomemixturesbehaveasasinglepurecompoundin correspondencetoaspecificcompositionandtemperatureatgivenpressures.Azeotropic compositionisfoundatconcentrationswherevolatilityisreversed,asshownin Fig.1.8.that representsthemixturesoftwopuresubstances, A and B.Intheleft-handzone,component A is morevolatilethancomponent B,whereasintheright-handzoneitistheopposite.Therefore
Fig.1.8
Liquidboilingpointsandvapourcondensationtemperaturesforminimum-boilingazeotrope mixturesofcomponents A and B
point Q intheliquid–vapourequilibriumzonecorrespondstoaliquidthatismorerichin B and toavapourthatismorerichin A thantheoriginalcomposition.Forpoint P itistheopposite.In correspondencetotheazeotropiccomposition,andtotheazeotropictemperature,vapourwill havethesamecompositionasliquid.
Table1.4 includessomeazeotropicmixtureswiththeindicationoftheazeotropiccomposition (firstcomponent)oftheazeotropictemperatureat1atm.
Table1.4Azeotropicmixturesat1atm(Dean,1999)
